Why your gas furnace may stop in a power outage

What uses electricity in a gas furnace

The gas burner makes the heat, but several electrical parts make the burner run safely and move warm air through the house:

• Blower motor. Pushes air across the heat exchanger and through your ducts. It is usually the largest electrical load in the furnace.
• Draft inducer. A smaller fan in many furnaces that moves combustion gases out through the vent before and during burning.
• Ignition. Many furnaces light the burner electronically rather than with a standing pilot flame.
• Controls and safety sensors. The control board, thermostat circuit, and flame and pressure sensors confirm that conditions are safe before and during operation.

If any of these lose power, a properly working furnace is designed to stay off. That is a safety feature, not a malfunction.

Why requirements vary so much

Furnaces differ in blower size, motor type, and staging. A furnace with a variable-speed blower may draw very different power from one with a single-speed motor, and the blower often runs at different speeds for heating, cooling, and fan-only modes. Some systems also power humidifiers, air cleaners, or zone dampers from the same circuit.

Because of that range, the gas furnace blower value in the calculator is only a rough starting point. How long the furnace runs each hour also depends on outdoor temperature, insulation, and your thermostat setting, so the duty cycle should be treated as an estimate.

How to find what your furnace needs

1. Find the furnace’s rating plate, usually inside the access panel. It lists the model number and electrical ratings such as volts and amps. Turn the thermostat off before opening the panel, and do not touch internal components.
2. Look up the installation or owner’s manual for that model number. The manufacturer’s documents describe electrical requirements and any guidance on backup power.
3. Ask an HVAC professional or the manufacturer whether your furnace is sensitive to the type of power produced by a particular inverter or generator. Some electronic controls are.
4. Find out how your furnace is connected. Many are hardwired to a dedicated switch rather than plugged into an outlet, which affects which backup options are possible.

Short checklist

• Record your furnace’s model number and electrical ratings.
• Find the installation or owner’s manual for that model.
• Find out whether the furnace is hardwired or plugged in.
• Ask an HVAC professional about compatibility with backup power.
• Test your carbon monoxide alarms and keep spare batteries.
• Plan a warm room and extra blankets in case heat is unavailable.
• Estimate energy needs with the calculator.

Sources and safety references

• CPSC safety tips to prevent carbon monoxide poisoning and fires (external, CPSC)
• Avoiding Carbon Monoxide Poisoning (external, CDC)
• Be Safe During a Power Outage (external, FEMA)
• Your furnace’s rating plate and installation or owner’s manual, for its exact electrical requirements.